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Frequently Asked Questions - Cancer

How do we track cancer incidence in MA?

The Massachusetts Cancer Registry (MCR) was established in 1980 and began collecting data on all cancer diagnoses on January 1, 1982. The MCR is a population-based comprehensive cancer surveillance system. MCR cancer incidence data, coupled with environmental data, can be used to evaluate potential relationships between cancer and the environment. The goal of tracking cancer incidence is to provide information that can be used to plan, evaluate, and take actions to prevent and control cancer in Massachusetts.

How does the Massachusetts Cancer Registry (MCR) collect information on new diagnoses of cancer in MA?

All hospitals in Massachusetts are required by law to report diagnoses of certain cases of malignant disease and benign brain-related tumors. In addition to hospitals, other facilities such as laboratories, radiation/oncology facilities, endoscopy centers, and dermatology and urology offices report to the MCR. Reports from dermatologists' offices and laboratories have only been collected by the MCR since 2001. Reports from urologists' offices have only been collected by the MCR since 2002. The MCR also collects information from reporting hospitals on individuals diagnosed and treated in staff physician offices when this information is available.

How do we evaluate different cancer types?

The term "cancer" is used to describe a variety of diseases associated with abnormal cell and tissue growth. Epidemiologic studies have revealed that different types of cancer are individual diseases with separate causes, risk factors, characteristics, and patterns of survival. Cancers are classified by the location in the body where the disease originated (the primary site) and the tissue or cell type of the cancer (histology). In general, cancer types should be evaluated separately. Cancers that occur as the result of the metastasis or the spread of a primary site cancer to another location in the body are not considered as separate cancers.

How do we calculate and interpret cancer incidence rates?

Direct Rates

Direct cancer incidence rates are used to directly compare rates for two or more different populations. Direct rates are calculated by applying age-specific cancer rates in an area to what epidemiologist refer to as a “standard population”. The U.S. population for the year 2000 is used by EPHT as the standard population. By applying the age-specific cancer rates for each area of interest to the same standard population, the direct rates can be compared to one another.  This method removes the differences between the areas that are due to the strong effect of age on the rate of cancer.

Direct cancer rates are one of the Nationally Consistent Data and Measures (NCDMs) used by all members of the Tracking program, and can be used to compare cancer incidence across counties and states.

Standardized Incidence Ratios

Standardized Incidence Ratios (SIRs) are used to compare cancer rates in smaller geographic areas to the cancer rate for the state as a whole.  These rates cannot be used to compare across geographies. SIRs are age-standardized and are calculated by applying age-specific cancer rates for Massachusetts as a whole to the age distribution of the area of interest.

SIRs are calculated by dividing the observed number of cancer diagnoses in an area of interest by the expected number of diagnoses for that area based on the statewide experience of that cancer type. The resulting ratio is multiplied by 100. A ratio of 100 indicates that the number of observed cancer diagnoses in a population is equal to the number that was expected.  An SIR greater than 100 indicates more diagnoses were observed than expected.  When an SIR is less than 100, fewer diagnoses were observed than expected.

Statistical significance is determined by a 95% confidence interval. A 95% CI is the range of estimated SIR values that has a 95% probability of including the true SIR for the population. If the 95% CI range does not include the value 100, then the study population is statistically significantly different from the comparison or "normal" population. "Statistically significantly different" means there is less than a 5% percent chance that the observed difference is the result of random fluctuation in the number of observed cancer diagnoses.

In addition to the range of the estimates contained in the confidence interval, the width of the confidence interval also reflects the stability of the SIR estimate. For example, a narrow confidence interval (e.g., 103-115) allows a fair level of certainty that the calculated SIR is close to the true SIR for the population. A wide interval (e.g., 85-450) leaves considerable doubt about the true SIR, which could be much lower than or much higher than the calculated SIR. This would indicate an unstable statistic. MA EPHT also incorporates the use of the Relative Standard Error (RSE) to assess statistical stability.  When the RSE is greater than 30%, the rate is unstable and caution should be exercised when interpreting results. In some circumstances with small numbers, an SIR may show as statistically significant and unstable.  In these cases, the results are unreliable.

What is an expected number?

The "expected" number of cancers refers to the number of diagnoses that would be expected in that community or census tract based on the cancer rate in Massachusetts as a whole. The age-specific statewide incidence rates are applied to the population distribution of each community to calculate the number of expected cancer diagnoses. The expected number reflects the number of diagnoses that would be expected to occur if the population of the community or census tract had the same cancer experience as a larger comparison population designated as "normal" or average. In this case, the state as a whole is selected to be the comparison population.

What is known about cancer and the environment?

From the National Institutes of Health's Cancer and the Environment: What You Need to Know, What You Can Do: Environmental factors such as viruses, sunlight, and chemicals interact with cells throughout our lives. Mechanisms to repair damage to our genes and healthy lifestyle choices (wearing protective clothing for sun exposure or not smoking) help to protect us from harmful exposures. However, over time, substances in the environment may cause gene alterations, which accumulate inside our cells. While many alterations have no effect on a person's health, permanent changes in certain genes can lead to cancer.

Because of the complex interplay of many factors, it is not possible to predict whether a specific environmental exposure will cause a particular person to develop cancer. We know that certain genetic and environmental factors increase the risk of developing cancer, but we rarely know exactly what combination of factors is responsible for a person's specific cancer. This also means that we usually don't know why one person gets cancer and another does not.

Additional information about cancer and the environment is available at:

What is the definition of a cancer cluster?

According to the U.S. Centers for Disease Control and Prevention (CDC), a cancer cluster is defined as a greater than expected number of cancer diagnoses that occurs within a group of people, in a geographic area, or over a period of time. In other words, the pattern of cancer appears unusual. A person may suspect that a cancer cluster exists when several loved ones, neighbors, or coworkers are diagnosed with cancer. However, what appears to be a cluster may actually reflect the expected number of cancer diagnoses within the group or area.

What factors are important in determining whether the pattern of cancer is unusual?

When considering whether the pattern of cancer in your area may be unusual, it is important to remember a few key facts:

  1. Cancer is a common disease, affecting about one in three people in their lifetime.
  2. The term cancer refers not to a single disease, but instead to a group of related yet different diseases.
  3. An apparent cancer cluster is more likely to be genuine if the diagnoses consist of one type of cancer, a rare type of cancer, or a type of cancer that is not usually found in an age group.

How are perceived cancer clusters evaluated?

The Massachusetts Department of Public Health's Bureau of Climate and Environmental Health uses a peer-reviewed protocol for community-specific environmental health assessments. Cancer incidence evaluations are available online for many regions and communities in Massachusetts.

How is a Geographic Information System used in cancer surveillance?

Geographic Information Systems (GIS) are used to assess exposure and health outcomes in environmental health tracking. Epidemiologists use GIS to evaluate the point pattern of cancer diagnoses in a geographic area on a map. In addition, other factors, such as proximity to an area of contamination or consideration of the population density are possible through the use of GIS-generated overlays.

The use of GIS allows epidemiologists to create maps that show the pattern of disease incidence in relation to environmental contamination in an area and identify potential exposure pathways.

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